Time-frequency interleaved mc-cdma for quasi-synchronous systems

ABSTRACT

The invention relates to digital transmissions. It particularly relates to a method of transmitting data from a transmitter to a receiver using multi-carrier Code Division Multiple Access (CDMA) for accessing a transmission system. The transmitted data are OFDM modulated using Orthogonal Frequency Division Multiplexing (OFDM) after being spread with a set of predefined spreading sequences of consecutive chips, wherein two successive chips of the predefined sequences are transmitted on non-successive carriers and in non-successive time intervals.

FIELD OF THE INVENTION

The invention generally relates to digital transmissions. In particular,it relates to a method of transmitting data using multi-carrier CodeDivision Multiple Access (CDMA) for accessing a transmission system andto a method of receiving such transmitted data.

The invention also relates to a transmission system, to a transmitterand to a receiver for carrying out the methods mentioned above.

It also relates to computer program products for carrying out suchmethods.

The invention generally applies to digital multi-user (multiple access)transmission systems and particularly to wireless and radio mobilecommunication systems such as e.g. next generation high data rate mobilecommunications systems (beyond 3^(rd) Generation).

BACKGROUND OF THE INVENTION

Due to the increasing demand for higher rate mobile data communications,the next generation cellular wireless systems, also called 4G systems,have the important challenge of providing high-capacityspectrum-efficient services to the customers. Therefore, even before thefull commercial deployment of 3G (3^(rd) Generation) systems, studiesand discussions on 4G systems (or IMT-2010+ systems) have alreadystarted. Efforts are being made to develop an air interface thatsupports the requirements of the increasing mobile data traffic.

Wideband Code Division Multiple Access (CDMA) systems have been proposedfor wireless communication networks. These systems provide higheraverage capacity and data rates than conventional multiple accesstechniques while spreading the data to be transmitted with predeterminedspreading sequences. Moreover, they are able to cope with theasynchronous nature of multimedia data traffic and enable combating thehostile channel frequency selectivity. However, the large frequencybandwidth of such high-speed wireless links makes them susceptible toInter Symbol Interference (ISI). Therefore, a number of multi-carrierCDMA techniques have been suggested to improve performance overfrequency selective channels. Multi-carrier CDMA combines the multipleaccess and cell reuse technology of CDMA systems with the robustnessagainst channel selectivity of multi-carrier systems using OrthogonalFrequency Division Multiplexing (OFDM). It is expected to be a majorcandidate for the physical layer of the 4G radio mobile system.Spreading can be performed either in the frequency domain, leading toMulti-Carrier CDMA (MC-CDMA), or in the time domain, leading toMulti-Tone CDMA (MT-CDMA) and Multi-Carrier Direct Sequence CDMA(MC-DS-CDMA).

The article by Hikmet Sari: “A Review of Multi-carrier CDMA”; publishedin the manual “Multi-Carrier Spread-Spectrum & Related Topics” by K.Fazel and S. Kaiser, Kluwer Academic Publishers, 2002, pages 3-12,mentions a system, which combines two variants of multi-carrier CDMAsystems, called “the two extremes”, wherein signal spreading isperformed either purely in the frequency domain, that is the MC-CDMAsystem, or in the time domain, that is the MC-DS-CDMA system,respectively. The combined system enables to create diversity both inthe time domain and in the frequency domain, by transmitting the chipsof a given symbol on a different carrier and in a different chip period.

Though the performance of this system may be better than the “twoextremes” mentioned, it is still not optimal with respect to quality(low interference and synchronism) upon reception.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a system, which yields abetter quality upon reception.

The invention takes the following aspects into consideration. Coherentdetection upon reception is facilitated if the data sent from varioustransmitters are received synchronously. In uplink transmissions,synchronism upon reception is very hard to obtain since the varioususers are generally not synchronized.

Therefore, the invention proposes a transmission scheme, which is morerobust to quasi-synchronism than the systems mentioned. To this end, amethod is proposed of transmitting data symbols using multi-carrier CodeDivision Multiple Access (MC-CDMA) for accessing a transmission system,the method comprising:

-   -   spreading the data symbols with a set of predefined spreading        sequences of successive chips for producing sequences of spread        data symbols including the data symbols multiplied by the chips,    -   mapping the spread data symbol sequences so that they are        assigned to selected sub-carriers among a set of predefined        sub-carriers and to selected time slots in a predefined periodic        time interval,    -   modulating the mapped spread data symbol sequences using        Orthogonal Frequency Division Multiplexing (OFDM) for producing        OFDM modulated symbols to be transmitted on the selected        sub-carriers and in the selected time slots,        wherein two successive spread data symbols are assigned to        non-successive sub-carriers and in non-successive time slots.

De-spreading upon reception after demodulation of the received OFDMsymbols leads to easily retrieving the expected encoded data sent byvarious users, whether synchronous or quasi-synchronous, since spreadingsequences allocated to the various users are supposed to benear-orthogonal, which implies that the correlation betweennon-successive spread data symbols of two distinct users is nearly zero.This allows finding the term representing the encoded data sent by eachdistinct user.

The transmission scheme of the invention is also more robust to channelselectivity both in time and frequency, since the spread data sequencesare distributed over on non-successive sub-carriers and time slots.Advantageously, this allows reducing interference upon reception andleads to better performance.

It is possible to use a unique scheme for uplink and downlinktransmissions. Only the mapping needs to be adapted to the system underconsideration.

By varying selected parameters, the invention also provides higherflexibility to the channel characteristics than known systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and additional features, which may be optionally used toimplement the invention to advantage, are apparent from and will beelucidated with reference to the drawings described hereinafter andwherein:

FIG. 1A and FIG. 1B are conceptual block diagrams illustrating examplesof a transmitter/method of transmission in accordance with theinvention, for uplink and downlink transmissions, respectively,

FIG. 2A and FIG. 2B are schematic diagrams illustrating two mappingexamples of a method of transmission in accordance with the invention,

FIG. 3A and FIG. 3B are schematic diagrams illustrating in detail themapping example illustrated in FIG. 2A for two different users,respectively,

FIG. 4A and FIG. 4B are conceptual block diagrams illustrating examplesof a receiver/method of reception in accordance with the invention, foruplink and downlink transmissions, respectively,

FIG. 5 is a conceptual block diagram illustrating an example of a systemin accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B show examples of a part of an MC-CDMA transmitter inaccordance with the invention. The transmission system can be anydigital multi-user transmission system, such as e.g. a radio mobilecommunication system. The proposed MC-CDMA scheme is particularlyadvantageous for the uplink transmissions (FIG. 1A) of a cellular systemdue to its asynchronous structure.

FIG. 1A illustrates an MC-CDMA transmitter in uplink transmissions. Itinvolves single user equipment e.g. a mobile phone sharing the samebandwidth with a number of users.

MC-CDMA transmission uses multi-carrier Code Division Multiple Access(MC-CDMA). A number of users, denoted Nu, sharing the same bandwidth areassigned predefined spreading codes to spread their data over the wholebandwidth of the channel. The spread data are sent at a set ofpredefined sub-carriers through the channel. In the example illustratedin FIG. 1A, the user of index k, k=1, . . . ,Nu, is assigned a specificspreading sequence of length L, of successive chips, denoted C_(k)^((i)), i=1, . . . ,L being the index of the chip in the sequence. Thespreading sequence is applied to input data symbols, denoted S_(k),which are actually already encoded by a source encoder and a channelencoder, not represented. Depending on the system, the spreadingsequences assigned to the various users may be orthogonal or nearorthogonal to each other but they must have predetermined properties.The number of sub-carriers and time slots for a given frame are denotedN_(c) and N_(t), respectively. For each user k, the transmitter of FIG.1A comprises:

-   -   spreading means SPREAD for spreading the incoming data symbols        S_(k) with the set of predefined spreading sequences (C_(k) ⁽¹⁾,        . . . ,C_(k) ^((L))), k=1, . . . ,Nu, of successive chips        assigned to user k for producing sequences of spread data        symbols including the data symbols multiplied by the chips,    -   mapping means MAP for mapping the spread data symbols sequences,        so that they are assigned to selected sub-carriers among a set        of N_(c) predefined sub-carriers and to selected time slots in a        predefined periodic time interval comprising N_(t) time slots,        so that two successive spread data symbols are assigned to        non-successive sub-carriers and in non-successive time slots,    -   modulating means OFDM for modulating the mapped spread data        symbol sequences using Orthogonal Frequency Division        Multiplexing (OFDM) for producing OFDM modulated symbols to be        transmitted on the selected sub-carriers and in the selected        time slots.

Serial-to-parallel S/P and parallel-to-serial P/S converters areprovided at the input of the spreader SPREAD and at the output of themapping means, respectively, in order to suitably organize the streamsof data for the following block operation. All users share the sametime-frequency mapping of chips. The spread data symbols are distributedboth on various selected sub-carriers and on various selected time slotscorresponding to a time-frequency interleaving, which enables to combatboth time and frequency selectivity of the channel. Moreover, twosuccessive spread data symbols are assigned to non-successivesub-carriers and in non-successive time slots, which enables to combateven better both time and frequency selectivity of the channel andadditionally leads to better robustness to quasi-synchronism. This willbe discussed in more detail below with reference to FIG. 3A and FIG. 3B.

Implementation details of the transmission method are given hereafter.For each user k, the serial to parallel converter S/P converts theincoming encoded data symbols S_(k) into a block of N_(c).N_(t)/Llow-rate parallel sub-streams, each of which being dedicated to modulateone of the N_(c) sub-carriers. The output of the serial to parallelconverter S/P feeds the spreader SPREAD of length L for spreading theincoming data symbol by the associated spreading waveform of user k,C_(k) ^((i)).

Then, mapping is performed to distribute the N_(c).N_(t) spread datasymbols on the corresponding time-frequency slots. At the mappingoutput, a parallel-to-serial block P/S guarantees that each block ofN_(c) spread symbols is an OFDM input symbol at a given time. Thereceived signal at the base station is the sum of all OFDM modulatedsignals coming from all users in the system transmitted through theirown channels.

FIG. 1B illustrates a transmitter in downlink transmissions inaccordance with the invention. The transmitter illustrated in FIG. 1Bmay be e.g. a base station of a radio mobile communication system, whichcommunicates with several users (downlink transmissions), denoted user lto user Nu. Most of the transmission chain is similar to thetransmission chain of FIG. 1A except that the outputs of the spreadersare summed before the mapping. The mapping is the same for all users. Atthe end of the transmission chain, the Nu sets of correspondingN_(c).N_(t) OFDM modulated spread symbols are sent through the channel.

FIG. 2 depicts two mapping matrix examples, which can be advantageouslyused with respect to the system used to implement the mapping step ofthe transmission method described above. The mapping example illustratedin FIG. 2A is well adapted to a system, wherein spreading sequences areorthogonal with respect to each other such as e.g. Walsh-Hadamardsequences. The mapping example illustrated in FIG. 2B is well adapted toa system wherein the spreading sequences have specific correlationproperties i.e. they have low inter-correlation and autocorrelationprofiles such as e.g. Gold sequences.

The number of sub-carriers and slots of a frame are given byN_(c)=K_(f).L and N_(t)=K_(t).L where K_(t) and K_(f) denoterespectively the time and frequency interleaving depths. The spreadingsequences are still of length L. Hence, each sub-matrix M_(i) ^(n) ofsize K_(t).K_(f) corresponds to the n^(th) chip of the spreadingsequence and contains K_(t).K_(f) data symbols chosen depending on thechannel, application and transmission characteristics. M_(i) ^(n) is notnecessarily a square matrix, and there are L×L sub-matrices M_(i) ^(n)so that the L chips of each of the K_(t).K_(f)L data symbols arerepresented. With such a mapping, K_(t).K_(f)L² spread data symbols aresimultaneously transmitted in the N_(c).N_(t) correspondingtime-frequency slots. The size of one OFDM symbol is still N_(c).

FIG. 2A illustrates a mapping example where the sub-matrices aresuccessively distributed in frequency, whereas FIG. 2B illustrates amapping example where the sub-matrices are successively distributed intime. In both cases, each spread data symbol is distributed on allsub-carriers and in all time slots of a frame, allowing the system tocombat efficiently both time and frequency selectivity of the channel.Finally, using the particular mapping of FIGS. 2A and e.g.Walsh-Hadamard spreading sequences, the system is robust to time offsetsof 0 to K_(t)−1 chips. Details about this are given below.

FIG. 3A and FIG. 3B represent an implementation example of the mappingmatrix of FIG. 2A, for two distinct users k and l, respectively, whichhave a time offset of 1 chip. In this example, K_(f)=K_(t)=2,N_(c)=N_(t)=8, L=4. The set of N_(c) sub-carriers, denoted f₁ to f₈ arerepresented on the horizontal axis, whereas the set of N_(t) time slots,denoted t₁ to t₈ are represented on the vertical axis. Incoming datasymbols of user k, denoted S_(k) ^(i), i=1, . . . ,16 and of user l,denoted S_(l) ^(j), j=1, . . . ,16, are grouped in four symbol-matrices,denoted m_(i)(k) and m_(i)(l), i=1, . . . ,4, respectively. For user k,the four symbol-matrices are: $\begin{matrix}{{m_{1}(k)} = \begin{pmatrix}S_{k}^{1} & S_{k}^{2} \\S_{k}^{3} & S_{k}^{4}\end{pmatrix}} & {{m_{2}(k)} = \begin{pmatrix}S_{k}^{5} & S_{k}^{6} \\S_{k}^{7} & S_{k}^{8}\end{pmatrix}} \\{{m_{3}(k)} = \begin{pmatrix}S_{k}^{9} & S_{k}^{10} \\S_{k}^{11} & S_{k}^{12}\end{pmatrix}} & {{m_{4}(k)} = \begin{pmatrix}S_{k}^{13} & S_{k}^{14} \\S_{k}^{15} & S_{k}^{16}\end{pmatrix}}\end{matrix}$Similarly, for user l the four symbol-matrices are the same as for userk, except that index k is replaced with index 1.

The spreading sequence of chips assigned to user k is denoted (C_(k)⁽¹⁾, C_(k) ⁽²⁾, C_(k) ⁽³⁾, C_(k) ⁽⁴⁾). The one assigned to user l isdenoted (C_(l) ⁽¹⁾, C_(l) ⁽²⁾, C_(l) ⁽³⁾, C_(l) ⁽⁴⁾). The mappingmatrices comprise L×L sub-matrices, denoted M_(i) ^(n)(k), i=1, . . .,L, of size K_(t)K_(f), where n=1 . . . L corresponds to the n^(th) chipof the spreading sequence, which sub-matrices comprise K_(t)K_(f)sub-matrix elements including the data symbols multiplied by thespreading sequence. Theses sub-matrices M_(i) ^(n)(k), i=1 . . . L, n=1. . . L, are, for user k: ${M_{i}^{1}(k)} = \begin{pmatrix}{S_{k}^{{4{({i - 1})}} + 1} \cdot C_{k}^{1}} & {S_{k}^{{4{({i - 1})}} + 2} \cdot C_{k}^{1}} \\{S_{k}^{{4{({i - 1})}} + 3} \cdot C_{k}^{1}} & {S_{k}^{{4{({i - 1})}} + 4} \cdot C_{k}^{1}}\end{pmatrix}$ ${M_{i}^{2}(k)} = \begin{pmatrix}{S_{k}^{{4{({i - 1})}} + 1} \cdot C_{k}^{2}} & {S_{k}^{{4{({i - 1})}} + 2} \cdot C_{k}^{2}} \\{S_{k}^{{4{({i - 1})}} + 3} \cdot C_{k}^{2}} & {S_{k}^{{4{({i - 1})}} + 4} \cdot C_{k}^{2}}\end{pmatrix}$ ${M_{i}^{3}(k)} = \begin{pmatrix}{S_{k}^{{4{({i - 1})}} + 1} \cdot C_{k}^{3}} & {S_{k}^{{4{({i - 1})}} + 2} \cdot C_{k}^{3}} \\{S_{k}^{{4{({i - 1})}} + 3} \cdot C_{k}^{3}} & {S_{k}^{{4{({i - 1})}} + 4} \cdot C_{k}^{3}}\end{pmatrix}$ ${M_{i}^{4}(k)} = \begin{pmatrix}{S_{k}^{{4{({i - 1})}} + 1} \cdot C_{k}^{4}} & {S_{k}^{{4{({i - 1})}} + 2} \cdot C_{k}^{4}} \\{S_{k}^{{4{({i - 1})}} + 3} \cdot C_{k}^{4}} & {S_{k}^{{4{({i - 1})}} + 4} \cdot C_{k}^{4}}\end{pmatrix}$

For user l, the L×L sub-matrices are the same as for user k, exceptindex k is replaced with index 1 and except that for user l, thesub-matrices are time shifted with an offset of one chip in the mappingmatrix, as shown in FIG. 3B. Therefore, the first line of the mappingmatrix of user l corresponding to the time slot t1 contains spread datasymbols of the last row of the previous mapping matrix, denoted S′_(l)^(i), i=15, 16, 11, 12, 7, 8, 3, 4, which does not correspond to thedata symbols S_(l) ¹ to S_(l) ¹⁶, since the sub-matrices are timeshifted.

With a time shift not exceeding K_(t)−1, this mapping scheme is morerobust to quasi-synchronism, since it allows retrieving the sent datasymbols more easily than known schemes, by making use of the correlationproperties of the orthogonal spreading sequences, that is:$\begin{matrix}{{\forall k},{\forall{l \neq k}}} \\{{\sum\limits_{i = 1}^{L}{C_{k}^{i}C_{l}^{i}}} = 0} \\{{\sum\limits_{i = 1}^{L}{C_{k}^{i} \cdot {C_{k}^{i}}^{*}}} = 1}\end{matrix}$

For example, de-spreading after demodulation at the receiver side, ofthe data symbols transmitted at frequency f₁ and in the time slot t₂,can be written as: $\begin{matrix}{{\frac{1}{4}{\sum\limits_{i = 1}^{4}{\left\lbrack {{S_{k}^{3} \cdot C_{k}^{i}} + {S_{l}^{1} \cdot C_{l}^{i}}} \right\rbrack \times {C_{k}^{i}}^{*}}}} = {{\frac{1}{4}S_{k}^{3}{\sum\limits_{i = 1}^{4}{C_{k}^{i} \cdot {C_{k}^{i}}^{*}}}} +}} \\{\frac{1}{4}S_{l}^{1}{\sum\limits_{i = 1}^{4}{C_{l}^{i} \cdot {C_{k}^{i}}^{*}}}} \\{= S_{k}^{3}}\end{matrix}$ since:${\sum\limits_{i = 1}^{4}{C_{k}^{i} \cdot {C_{k}^{i}}^{*}}} = 1$ and:${\sum\limits_{i = 1}^{4}{C_{k}^{i}{C_{l}^{i}}^{*}}} = 0$

Therefore, using a particular mapping in accordance with the inventionenables to cope with quasi-synchronism. Actually, the example describedabove allowing retrieving S_(k) ³ only works well for K_(t)×L/2 symbols,that is one line out of 2 in the mapping matrix example of FIG. 3A andFIG. 3B. In all other cases, the results are not exactly equal to theexpected data symbols but lead to partial sums with residual terms.These residual terms are easy to eliminate afterwards. Using largeenough sub-matrices, the number of cases where the calculations lead toresidual terms in addition to the expected data symbols is reduced.Using such sub-matrices also reduces interference due to the occurrenceof partial sums, which improves performance.

FIG. 4 shows two examples of MC-CDMA receivers in accordance with theinvention. FIG. 4A illustrates e.g. a base station receiver of a mobiletransmission system in uplink transmissions. The base station receivesdata encoded by several user equipments of index 1 to Nu, sent via theMC-CDMA mobile transmission system, which uses multi-carrier CodeDivision Multiple Access (CDMA) and OFDM modulation. The receivedencoded data are spread with a set of predefined spreading sequences oflength L assigned to the various users, denoted (C_(k)(1), . . .,C_(k)(L)), k being the index of the considered user concerned. Thereceiver comprises at least:

-   -   a demodulator OFDM⁻¹ for demodulating the received multi-carrier        data with respect to a set of predefined sub-carriers,    -   de-mapping means MAP⁻¹ for de-mapping the demodulated data and        for retrieving the set of predefined spreading sequences and    -   de-spreading means SPREAD⁻¹ for de-spreading the set of        predefined spreading sequences for retrieving the encoded data        sent by the transmitter.

Serial-to-parallel S/P and parallel-to-serial P/S converters areprovided at the output of the demodulator OFDM⁻¹ and the de-spreadingmeans SPREAD⁻¹, respectively, in order to suitably organize the outputstream of data for the following block operation. At the end of thereceiving chain, decoding means DECOD are represented to indicate thatthe receiver finally needs to decode (source decoding and channeldecoding) the de-spread data to retrieve the original data message sentby the transmitter.

FIG. 4B illustrates e.g. a user equipment receiver in downlinktransmissions of a mobile communications system. Like block elements asin the receiver of FIG. 4A are indicated by like reference letters.During downlink transmissions, the user equipment of index k only has tode-spread the data sent by the base station and which are destined toits own decoder. Therefore, the user equipment of user k only has toknow the spreading sequence of user k that is (C_(k)(1), . . .,C_(k)(L)).

FIG. 5 shows a system in accordance with the invention comprising atransmitter 51, a receiver 52 and a transmission channel 53, fortransmitting data from the transmitter to the receiver via thetransmission channel. Depending on the system and the kind oftransmissions performed, the transmitter and receiver may alternativelybe the same devices. In a mobile communication system, typically, theuser equipment would be the receiver and the base station would be thetransmitter during downlink transmissions, whereas in uplinktransmissions, the base station would be the receiver and the userequipment the transmitter. In uplink transmissions, the transmitter maybe similar in design to the MC-CDMA transmitter depicted in FIG. 1A, andthe receiver may be similar in design to the MC-CDMA receiver depictedin FIG. 4A. In downlink transmissions, the transmitter may be of similardesign to the MC-CDMA transmitter depicted in FIG. 1B and the receivermay be of similar design to the MC-CDMA receiver depicted in FIG. 4B.

The drawings and their description hereinbefore illustrate rather thanlimit the invention. It will be evident that there are numerousalternatives, which fall within the scope of the appended claims. Inthis respect, the following closing remarks are made.

There are numerous ways of implementing functions by means of items ofhardware or software, or both. In this respect, the drawings are verydiagrammatic, each representing only one possible embodiment of theinvention. Thus, although a drawing shows different functions asdifferent blocks, this by no means excludes that a single item ofhardware or software carries out several functions. Nor does it excludethat an assembly of items of hardware or software, or both carries outone function.

Any reference sign in a claim should not be construed as limiting theclaim. Use of the verb “to comprise” and its conjugations does notexclude the presence of elements or steps other than those stated in aclaim. Use of the article “a” or “an” preceding an element or step doesnot exclude the presence of a plurality of such elements or steps.

1. Method of transmitting data symbols using multi-carrier Code DivisionMultiple Access (MC-CDMA) for accessing a transmission system, themethod comprising: spreading the data symbols with a set of predefinedspreading sequences of successive chips for producing sequences ofspread data symbols including the data symbols multiplied by the chips,mapping the spread data symbol sequences so that they are assigned toselected sub-carriers among a set of predefined sub-carriers and toselected time slots in a predefined periodic time interval, modulatingthe mapped spread data symbol sequences using Orthogonal FrequencyDivision Multiplexing (OFDM) for producing OFDM modulated symbols to betransmitted on the selected sub-carriers and in the selected time slots,wherein two successive spread data symbols are assigned tonon-successive sub-carriers and in non-successive time slots.
 2. Methodas claimed in claim 1, wherein the step of mapping includes defining amapping matrix of size K_(t)L×K_(f)L, L being the length of thepredefined spreading sequences, K_(t) and K_(f) denoting time andfrequency interleaving depths respectively, K_(t)L representing thenumber of time slots in the periodic time interval and K_(f)Lrepresenting the number of sub-carriers in the set of predefinedsub-carriers, an OFDM modulated symbol being transmitted in a time slotand transporting K_(f)L spread data symbols, wherein the mapping matrixcomprises L×L sub-matrices, denoted M_(i) ^(n), i=1 . . . L, of sizeK_(t)K_(f), where n=1 . . . L corresponds to the n^(th) chip of thespreading sequence, which sub-matrices comprise K_(t)K_(f) sub-matrixelements corresponding to spread data symbols, for simultaneouslytransmitting K_(t)K_(f)L² spread data symbols on the correspondingselected sub-carriers and in the corresponding selected time slots andwherein the positions of the sub-matrix elements are predetermined withrespect to quality criteria depending on the transmission system. 3.Method as claimed in claim 2, wherein the sub-matrices are distributedin the mapping matrix in order that the sub-matrices M_(i) ^(n)corresponding to a same n^(th) chip are assigned to same set of K_(f)successive sub-carriers.
 4. Method as claimed in claim 2, wherein thesub-matrices are distributed in the mapping matrix in order that thesub-matrices M_(i) ^(n) corresponding to a same n^(th) chip are assignedto a same set of K_(t) successive time slots.
 5. Transmitter fortransmitting data symbols using multi-carrier Code Division MultipleAccess (CDMA) for accessing a transmission system, comprising: spreadingmeans for spreading the data symbols with a set of predefined spreadingsequences of successive chips for producing sequences of spread datasymbols including the data symbols multiplied by the chips, mappingmeans for mapping the spread data symbol sequences so that they areassigned to selected sub-carriers among a set of predefined sub-carriersand to selected time slots in a predefined periodic time interval,modulating means for modulating the mapped spread data symbol sequencesusing Orthogonal Frequency Division Multiplexing (OFDM) for producingOFDM modulated symbols to be transmitted on the selected sub-carriersand in the selected time slots, wherein two successive spread datasymbols are assigned to non-successive sub-carriers and innon-successive time slots.
 6. Method of receiving multi-carrier dataencoded by a transmitter and sent via a transmission system usingmulti-carrier Code Division Multiple Access (CDMA) for accessing thetransmission system, the encoded data being OFDM modulated after beingspread with a set of predefined spreading sequences, the methodcomprising: demodulating the received multi-carrier data with respect toa set of predefined sub-carriers, de-mapping the demodulated data forretrieving the set of predefined spreading sequences and de-spreadingthe set of predefined spreading sequences for retrieving the encodeddata sent by the transmitter.
 7. Receiver for receiving data encoded bya transmitter and sent via a transmission system using multi-carrierCode Division Multiple Access (CDMA) for accessing the transmissionsystem, the data being OFDM modulated after being spread with a set ofpredefined spreading sequences, the receiver comprising: a demodulatorfor demodulating the received multi-carrier data with respect to a setof predefined sub-carriers, de-mapping means for de-mapping thedemodulated data for retrieving the set of predefined spreadingsequences and de-spreading means for de-spreading the set of predefinedspreading sequences for retrieving the encoded data sent by thetransmitter. Computer program product for a transmitter computing a setof instructions, which when loaded into the transmitter, causes thetransmitter to carry out the method as claimed in claim
 1. 8. Computerprogram product for a receiver computing a set of instructions, whichwhen loaded into the receiver, causes the receiver to carry out themethod as claimed in claim
 6. 9. Transmission system comprising at leasta transmitter and a receiver for transmitting data from the transmitterto the receiver using multi-carrier Code Division Multiple Access (CDMA)for accessing the transmission system, the transmitted data beingmodulated using Orthogonal Frequency Division Multiplexing (OFDM) afterbeing spread with a set of predefined spreading sequences of consecutivechips, wherein two successive chips of the predefined sequences aretransmitted on non-successive carriers and in non-successive timeintervals.